Techniques to remotely measure crop status have historically include the use of a spectroradiometer and other instruments (Bausch et al. 1994; Chappelle et al. 1992; Maas and Dunlap, 1989), aerial photography (Benton et al, 1976), and satellite imagery.
The techniques listed above are not without their limitations. For example, early research by Resource21 ™ determined that during the optimal fly over times between 10 a.m. and 11 a.m. for satellite imaging, cloud cover had adverse affects on visibility. It was found that during the 10 a.m. to 11 a.m. time frame, fields in Colorado were visible approximately 80% of the time while eastern Nebraska fields were visible approximately 50% of the time. This trend in decreased visibility continued the farther east that data was collected. Also, spatial resolution for satellite imagery is poor (Landsat, 20 meter and panchromatic, 10 meter). Similar problems plague aerial photographic methods as well. While aerial imagery has better spatial resolution (typically less than 3 meters) than satellite imaging, partial cloud cover can shade sections of fields giving biased or incorrect reflectance measurements. Both techniques, however, suffer from the need for extensive data processing (performed by third party providers at high cost and long lead time) and geo-referencing issues. Even with spectroradiometric methods using sunlight as the ambient light source, cloud cover and time of day (8 a.m. to 8 p.m.) demands limit the mainstream acceptance of the technology for addressing the nitrogen rate over-loading problem.
Vehicle-mounted, active sensing technologies overcome the limitations of the passive technologies listed above by utilizing artificially generated light to irradiate a plant canopy and measure a portion of this light that is reflected off the canopy, much like the passive sensing instrumentation. Active sensors can have either steady-state or modulated light sources. With steady-state light illumination, care must be taken to adequately shield the measurement scene (typically a leaf) from ambient light such as in the case of spectrophotometric measurements utilizing a halogen lamp. However, sensors with modulated light sources can be operated without concern for ambient background illumination. With a modulated active sensor, the modulated radiation is reflected from the target and measured by the sensor's detection hardware. Electrical circuits within the sensor are able to differentiate between the modulated portion of the reflectance and ambient background light. This unique feature of active sensors is why they can operate equally well under all lighting conditions. Active sensors are sometimes referred to as real-time or on-the-go sensors. This simply means that the data or measurements produced by the sensors can be utilized immediately for performing agricultural operations such as applying herbicide or fertilizer.
Active plant canopy sensors have a long history dating back almost 70 years. One of the earliest active electro-optical sensors was developed by Ferté and Balp (U.S. Pat. No. 2,177,803). This sensor was designed to be spectrally sensitive to a plant's carotenoid peak located at 550 nm for the intended purpose of detecting plants and selective thinning. The detection system utilized two phototubes each fitted with spectrally selective filters. One filter was colored with methyl green pigment to give the associated detector a spectral sensitivity to vegetation with a spectral peak located at 535 nm while the other filter was colored with rhodamine B to create a notch filter to block green light. The interplay between the optical signals sensed by the detection circuitry was utilized by the system to activate a plant thinning device.
Another early active sensor was developed by Marihart (U.S. Pat. No. 2,682,132). This particular sensor was vehicle mounted and was developed for the selective application of herbicides and fertilizer. The sensing system utilized a modulated light source consisting of a fluorescent lamp and a phototube connected to an inductor-capacitor tuned amplifier to measure light reflected from the plant canopy. Spectral selectivity was performed via the use of color filters in front of either the detector or the light source.
In 1969, Palmer et al. developed a sugar beet singling and thinning system to automatically thin plant populations. This instrument incorporated four optically modulated sensors connected to a PDP-8 minicomputer mounted to a tractor/mower. Plant distribution was detected via two photomultiplier tubes fitted with optical band pass filters inside each sensor. The center wavelengths for the filters were 630 nm and 810 nm with each filter having an apparent bandwidth of roughly 60 nm. The minicomputer was programmed to create a 2-dimensional “kill map” of plants to be eliminated. When a plant to be eliminated was detected, the system would respond by spraying the plant with a non-selective herbicide.
During the time period spanning from 1975 to 2002, fully solid-state plant status and weed sensors were developed. These sensors utilized LED's to actively illuminate plant canopies in order to overcome the limitations of lamp-based and passive illumination methods. Henderson and Grafton (U.S. Pat. No. 3,902,701) developed one of the first active sensor instruments to use LED's as an illumination source. The instrument was designed to be mobile with an intended use to measure leaf reflectance characteristics and relate this reflectance to plant health and status. Stafford et al. (1989) developed a portable handheld sensor to measure turf moisture content. This instrument contained two near infrared (NIR) monochromatic LED's with one LED source emitting 940 nm light and the other 1150 nm light. Subsequently, Beck and Vyse (U.S. Pat. No. 5,292,702) developed an active weed sensor, much like the Henderson sensor, incorporating two LED light sources with one LED source emitting 670 nm light and the other 750 nm light (WeedSeeker by Patchen, Ukiah, Calif.). Stone et al. (U.S. Pat. No. 6,596,996) developed a dual wavelength active light sensor, essentially a form of the Henderson and Beck patents, for the purpose of quantitative biomass determination while Holland (U.S. Pat. No. 7,408,145) developed a plant biomass sensor utilizing a novel polychromatic LED light source. For all the aforementioned solid state sensors, the light sources were modulated and detected reflectance signals were demodulated synchronously. Reusch in European Patent EP 1 429 594 and his paper submitted to the 6th European Conference on Precision Agriculture discloses a technique to use halogen and flash lamp techniques for active illumination. The instrumentation taught in these documents are similar to the instrument disclosed by Palmer and Owens (1969). It should be noted concerning spectral selectivity, that Holland (U.S. Pat. No. 7,408,145) describes a device that addresses the concerns Reusch has argued regarding LED technology.